Semiconductor memory device having complete hidden refresh function

ABSTRACT

In a DRAM having a complete hidden refresh function, when data refresh is to be carried out in an active mode, a signal for selecting a way is set to an “H” level and then reset to an “L” level at each cycle while the corresponding upper address is designated. When data refresh is to be carried out in a standby mode, the signal for selecting the way is maintained at an “H” level and is not reset to an “L” level while the corresponding upper address is designated. This can reduce the standby current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device, andparticularly to a semiconductor memory device having a complete hiddenrefresh function for performing data refresh without being externallyinstructed.

2. Description of the Background Art

In the field of portable terminals such as portable telephones, anasynchronous general-purpose static semiconductor memory device(hereinafter, referred to as “SRAM”) for which external clocks need notbe supplied is widely used. Since the SRAM does not require datarefresh, complex control is unnecessary. For example, it is unnecessaryto control such that an access to the memory is awaited until a refreshcycle is finished. For this reason, the use of the SRAM can simplify thesystem configuration, and therefore, the SRAM is suitable for use withthe portable terminal.

Recently, the function of the portable terminal has been improvedsignificantly, and the terminal requires a large-capacity memory. TheSRAM however has a memory cell size that is about 10 times that of adynamic semiconductor memory device (hereinafter, referred to as“DRAM”). When a large-capacity SRAM is used, the cost for the memorychip is significantly increased, and consequently, the price of theportable terminal is increased. To overcome the problem, a new technicalscheme was conceived in which, instead of the SRAM, a DRAM lower in costper unit bit is used for the portable terminal.

The DRAM however requires complex memory control for the refreshoperation. For portable-terminal manufacturers that hitherto have beenengaged in design of systems using SRAMs as memories, it is not easy touse DRAMs as substitutive memories of SRAMs. Under these circumstances,many semiconductor manufacturers have begun the development of a newsemiconductor memory device that is formed of a DRAM but operates as anSRAM in terms of external functions

In this new semiconductor memory device, the same memory cells as thoseused in the DRAM are used. On the other hand, external interfaces, suchas control signals and address signals to be input to the semiconductormemory device, are the same as those to be input to the SRAM. However,different from the refresh operation or the self-refresh operation ofthe conventional DRAM, the refresh operation of the new semiconductormemory device is not controlled by signals received from an externalsource. Rather, the refresh operation is controlled by a refreshinstruction signal /REFE that is periodically output from a refreshcircuit provided in the semiconductor memory device (for example, seeJapanese Patent Laying-Open No. 2002-352577). The new semiconductormemory device capable of such completely hidden refresh will behereinafter referred to as a “complete hidden refresh function-equippedDRAM”. The development of the complete hidden refresh function-equippedDRAM offers support to improved functions of the portable terminals.

There is also proposed a DRAM reduced in power consumption, wherein in aself-refresh mode, an upper address is assigned to each of ways (wordline groups) and a lower address is assigned to each word line belongingto each of the ways, and while one way is selected, the way selectionsignal for selecting the way is maintained at an active level to preventreset of the signal to an inactive level (for example, see JapanesePatent Laying-Open No. 09-161477).

In the complete hidden refresh function-equipped DRAM as well, it isimportant to reduce power consumption, since it is used for a portableterminal.

With the complete hidden refresh function-equipped DRAM, however, datarefresh is carried out in response to refresh instruction signal /REFEeven in the active mode. This means that the read/write operation andthe refresh operation co-exist in the active mode, making it impossibleto fix the way selection signal to an active level. Further, the wayselection signal is reset to an inactive level every time a loweraddress is changed even in the standby mode, for the purpose of matchingwith the active mode. This increases the consumed current.

SUMMARY OF THE INVENTION

In view of the foregoing, a main object of the present invention is toprovide a semiconductor memory device having a complete hidden refreshfunction and reduced in power consumption.

A semiconductor memory device according to the present invention is asemiconductor memory device having a complete hidden refresh functionfor performing data refresh without being externally instructed, whichincludes: a memory cell array having a plurality of memory cellsarranged in rows and columns, a plurality of word lines providedcorresponding to the respective rows, and a plurality of bit line pairsprovided corresponding to the respective columns, the word lines beingdivided into a plurality of word line groups; an oscillator foroutputting a clock signal having a predetermined first period; a groupselection circuit operative in synchronization with the clock signal,and for sequentially selecting the plurality of word line groups at asecond period that is a multiple of the first period, the groupselection circuit setting a group selection signal indicating theselected word line group to an active level and then resetting the sameto an inactive level at the first period in an active mode where dataread/write can be performed, and setting the group selection signal toan active level and then resetting the same to an inactive level at thesecond period in a standby mode where data is retained; a word lineselection circuit for sequentially selecting a plurality of word linesbelonging to the word line group selected by the group selection circuitat the first period, the word line selection circuit setting a word lineselection signal indicating the selected word line to an active leveland then resetting the same to an inactive level at the first period; aword driver provided corresponding to each word line, and for settingthe corresponding word line to a selected level when both of thecorresponding group selection signal and the corresponding word lineselection signal are set to the active level; and a refresh executingcircuit for carrying out data refresh of each memory cell correspondingto the word line set to the selected level by the word driver.

Accordingly, during the period in which one word line group is selectedin the standby mode, the group selection signal is maintained at anactive level, without being reset to an inactive level. This can reducepower consumption compared to the conventional case where the groupselection signal would be reset to an inactive level every time a wordline is selected.

Another semiconductor memory device according to the present inventionis a semiconductor memory device having a complete hidden refreshfunction for performing data refresh without being externallyinstructed, which includes: a memory cell array divided into a pluralityof memory blocks, each memory block having a plurality of memory cellsarranged in rows and columns, a plurality of word lines providedcorresponding to the respective rows, and a plurality of bit line pairsprovided corresponding to the respective columns; an oscillator foroutputting a clock signal having a predetermined first period; a wordline selection circuit operative in synchronization with the clocksignal, and for sequentially selecting the plurality of word lines at asecond period that is a multiple of the first period, the word lineselection circuit setting a word line selection signal indicating theselected word line to an active level and then resetting the same to aninactive level at the first period in an active mode where dataread/write can be performed, and setting the word line selection signalto an active level and then resetting the same to an inactive level atthe second period in a standby mode where data is retained; a blockselection circuit for sequentially selecting the plurality of memoryblocks at the first period, the block selection circuit setting a blockselection signal indicating the selected memory block to an active leveland then resetting the same to an inactive level at the first period; aword driver provided corresponding to each word line, and for settingthe corresponding word line to a selected level when both of thecorresponding word line selection signal and the corresponding blockselection signal are set to the active level; and a refresh executingcircuit for carrying out data refresh of each memory cell correspondingto the word line set to the selected level by the word driver.

Accordingly, during the period in which one word line is selected in thestandby mode, the word line selection signal is maintained at an activelevel, without being reset to an inactive level. Therefore, powerconsumption is reduced compared to the conventional case where the wordline selection signal would be reset to an inactive level every time amemory block is selected.

Yet another semiconductor memory device according to the presentinvention is a semiconductor memory device having a complete hiddenrefresh function for performing data refresh without being externallyinstructed, which includes: a memory cell array divided into a pluralityof memory blocks, each memory block having a plurality of memory cellsarranged in rows and columns, a plurality of word lines providedcorresponding to the respective rows, and a plurality of bit line pairsprovided corresponding to the respective columns; a refresh executingcircuit provided between every two of the plurality of memory blocks,and for carrying out data refresh of each memory cell corresponding to aword line set to a selected level in the adjacent memory blocks; anoscillator for outputting a clock signal having a predetermined firstperiod; a block selection circuit operative in synchronization with theclock signal, and for sequentially selecting the plurality of memoryblocks at a second period that is a multiple of the first period, theblock selection circuit setting a block selection signal indicating theselected memory block to an active level and then resetting the same toan inactive level at the first period in an active mode where dataread/write can be performed, and setting the block selection signal toan active level and then resetting the same to an inactive level at thesecond period in a standby mode where data is retained; a word lineselection circuit for sequentially selecting a plurality of word linesbelonging to the memory block selected by the block selection circuit atthe first period, the word line selection circuit setting a word lineselection signal indicating the selected word line to an active leveland then resetting the same to an inactive level at the first period; aconnection circuit provided corresponding to each memory block, and forconnecting the corresponding memory block to the corresponding refreshexecuting circuit and disconnecting the other memory block from therelevant refresh executing circuit while the corresponding blockselection signal is at the active level; and a word driver providedcorresponding to each word line, and for setting the corresponding wordline to a selected level when both of the corresponding block selectionsignal and the corresponding word line selection signal are set to theactive level.

Accordingly, during the period in which one memory block is selected inthe standby mode, the block selection signal is maintained at an activelevel, without being reset to an inactive level. This can reduce powerconsumption compared to the conventional case where the block selectionsignal is reset to an inactive level every time a word line is selected.

As described above, according to the present invention, it is possibleto reduce consumed power of a semiconductor memory device having acomplete hidden refresh function.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an overall configuration of a completehidden refresh function-equipped DRAM according to a first embodiment ofthe present invention.

FIG. 2 is a block diagram showing layout of a row decoder and a memorymat shown in FIG. 1.

FIG. 3 is a circuit diagram showing details of a main part of FIG. 2.

FIG. 4 is a circuit block diagram showing a configuration of a refreshinstruction circuit included in a control circuit shown in FIG. 1.

FIG. 5 is a circuit block diagram showing a configuration of aninstruction signal activating circuit shown in FIG. 4.

FIG. 6 is a circuit block diagram showing a configuration of adetermination circuit shown in FIG. 4.

FIG. 7 is a timing chart illustrating an operation of the refreshinstruction circuit shown in FIGS. 4-6.

FIG. 8 is another timing chart illustrating the operation of the refreshinstruction circuit shown in FIGS. 4-6.

FIG. 9 is a circuit block diagram showing a portion of the row decoderof FIG. 1 related to a refresh operation.

FIG. 10 is a block diagram showing a configuration of an addressgenerating circuit shown in FIG. 9.

FIG. 11 is a circuit block diagram showing a configuration of a latchcircuit 98 shown in FIG. 9.

FIG. 12 is a circuit block diagram showing a configuration of a latchcircuit 99 shown in FIG. 9.

FIG. 13 is a timing chart illustrating an operation of the circuitportion shown in FIG. 9.

FIG. 14 is a block diagram showing layout of a row decoder and a memorymat of a complete hidden refresh function-equipped DRAM according to asecond embodiment of the present invention.

FIG. 15 is a circuit diagram showing a main part of the row decoder andthe memory mat shown in FIG. 14.

FIG. 16 is a circuit block diagram showing a portion of the row decoderof FIG. 14 related to the refresh operation.

FIG. 17 is a timing chart illustrating an operation of the circuitportion shown in FIG. 16.

FIG. 18 is a block diagram showing layout of a row decoder and a memorymat of a complete hidden refresh function-equipped DRAM according to athird embodiment of the present invention.

FIG. 19 is a circuit diagram showing a main part of a word driver groupshown in FIG. 18.

FIG. 20 is a circuit block diagram showing a portion of the row decoderof FIG. 18 related to the refresh operation.

FIG. 21 is a block diagram showing a configuration of an addressgenerating circuit shown in FIG. 20.

FIG. 22 is a timing chart illustrating an operation of the circuitportion shown in FIG. 20.

FIG. 23 is a circuit block diagram showing a portion of a row decoderrelated to the refresh operation in a complete hidden refreshfunction-equipped DRAM according to a fourth embodiment of the presentinvention.

FIG. 24 is a timing chart illustrating an operation of the circuitportion shown in FIG. 23.

FIG. 25 is a circuit block diagram showing a main part of a completehidden refresh function-equipped DRAM according to a fifth embodiment ofthe present invention.

FIG. 26 is a circuit block diagram showing a main part of a completehidden refresh function-equipped DRAM according to a sixth embodiment ofthe present invention.

FIG. 27 is a timing chart illustrating an operation of the circuitportion shown in FIG. 26.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a block diagram showing an overall configuration of a completehidden refresh function-equipped DRAM according to a first embodiment ofthe present invention. In FIG. 1, this complete hidden refreshfunction-equipped DRAM includes a control signal input terminal group 1,a column address input terminal group 2, a row address input terminalgroup 3, a lower data input/output terminal group 4, an upper datainput/output terminal group 5, a power supply terminal 6, and a groundterminal 7. Control signal input terminal group 1 receives a chip enablesignal /CE, an output enable signal /OE, a write enable signal /WE, andcontrol signals /LB and /UB. Column address input terminal group 2receives column address signals A0-Ai (i is an integer not lower than0). Row address input terminal group 3 receives row address signals Ai+1to Aj (j is a natural number). Lower data input/output terminal group 4allows lower data signals DQ0-DQ7 to be input/output therethrough. Upperdata input/output terminal group 5 allows upper data signals DQ8-DQ15 tobe input/output therethrough. Power supply terminal 6 receives a powersupply potential VCC. Ground terminal 7 receives aground potential GND.

Chip enable signal /CE sets the complete hidden refreshfunction-equipped DRAM to an active mode. Output enable signal /OE setsthe complete hidden refresh function-equipped DRAM to a read mode andconcurrently activates an output buffer. Write enable signal /WE setsthe complete hidden refresh function-equipped DRAM to a write mode.Control signal /LB selects an operation of inputting/outputting lowerdata signals DQ0-DQ7. Control signal /UB selects an operation ofinputting/outputting upper data signals DQ8-DQ15.

The complete hidden refresh function-equipped DRAM further includes acontrol circuit 10, a column address buffer 11, and a row address buffer12. Control circuit 10 generates various internal control signalsaccording to control signals /CE, /OE, . . . externally provided viacontrol signal input terminal group 1, and sets the complete hiddenrefresh function-equipped DRAM to a prescribed operation mode such as awrite mode, a read mode, or a refresh mode. Column address buffer 11receives column address signals A0-Ai externally provided via columnaddress input terminal group 2, and transfers them to the inside. Rowaddress buffer 12 receives row address signals Ai+1 to Aj externallyprovided via row address input terminal group 3, and transfers them tothe inside.

The complete hidden refresh function-equipped DRAM further includes acolumn decoder 13, a row decoder 14, a memory cell array 15, and a senseamplifier+input/output controller circuit 16. Column decoder 13specifies a column address according to column address signals A0-Aiprovided from column address buffer 11. Row decoder 14 specifies a rowaddress according to row address signals Ai+1 to Aj provided from rowaddress buffer 12. Row decoder 14 also generates an internal row addresssignal in response to a refresh instruction signal /REFE from controlcircuit 10, and specifies a row address according to the generatedinternal row address signal. Memory cell array 15 includes a pluralityof memory cells arranged in rows and columns. Senseamplifier+input/output controller circuit 16 performs data refresh ofthe memory cells, data write to the memory cells, and data read from thememory cells. Memory cell array 15 and sense amplifier+input/outputcontroller circuit 16 constitute a memory mat 17.

The complete hidden refresh function-equipped DRAM further includes alower input buffer 18, a lower output buffer 19, an upper input buffer20, and an upper output buffer 21. Lower input buffer 18 receives lowerwrite data signals D0-D7 via lower data input/output terminal group 4,and transfers them to sense amplifier+input/output controller circuit16. Lower output buffer 19 receives lower read data signals Q0-Q7 fromsense amplifier+input/output controller circuit 16, and outputs them tolower data input/output terminal group 4. Upper input buffer 20 receivesupper write data signals D8-D15 via upper data input/output terminalgroup 5, and transfers them to sense amplifier+input/output controllercircuit 16. Upper output buffer 21 receives upper read data signalsQ8-Q15 from sense amplifier+input/output controller circuit 16, andoutputs them to upper data input/output terminal group 5. Senseamplifier+input/output controller circuit 16 and buffers 18-21 areconnected via a global signal input/output line pair group G10.

FIG. 2 is a diagram showing layout of row decoder 14 and memory mat 17shown in FIG. 1. Referring to FIG. 2, this complete hidden refreshfunction-equipped DRAM uses a so-called alternate shared sense amplifiermethod. More specifically, memory array 15 is divided into a pluralityof memory blocks BK1-BKm (m is an integer of 2 or greater), senseamplifier+input/output controller circuit 16 is divided into a pluralityof sense amplifier bands SA0-SAm, and memory blocks BK1-BKm arerespectively located between every two of sense amplifier bands SA0-SAm.

Sense amplifier band SA0 is provided with a plurality of senseamplifiers 22 corresponding, for example, to even columns of adjacentmemory block BK1. Sense amplifier band SA1 is provided with a pluralityof sense amplifiers 22 corresponding, e.g., to odd columns of adjacentmemory blocks BK1 and BK2. Sense amplifiers 22 of sense amplifier bandSA1 are shared by memory blocks BK1 and BK2. Determination as to whichsense amplifier 22 of sense amplifier band SA1 is to be used by whichone of memory blocks BK1 and BK2 is made based on signals BLIL1 andBLIR1 input from row decoder 14. Other sense amplifier bands SA2-SAmeach have a similar configuration.

Row decoder 14 includes a plurality of word driver groups WD1-WDm. Worddrivers WD1-WDm are provided corresponding to memory blocks BK1-BKm,respectively. Word driver group WD1 selects one row of memory block BK1in response to a signal group X and signals RX0-1 and RX1-1. SignalsBLIL1, BLIR1, signal group X, and signals RX0-1, RX1-1 are generated inrow decoder 14 based on row address signals Ai+1 to Aj and others. Otherword driver groups WD2-WDm each have a similar configuration.

FIG. 3 is a circuit block diagram partially showing a structure ofmemory block BK1 of FIG. 2 and the periphery thereof. Referring to FIG.3, memory block BK1 includes a plurality of memory cells MCs arranged inrows and columns, a plurality of word lines WLs provided correspondingto the respective rows, and a plurality of bit line pairs BLP providedcorresponding to the respective columns. Memory cell MC includes anaccess MOS transistor Q and an information storage capacitor C. Wordline WL transmits an output of word driver group WD1 and activatesmemory cell MC in a selected row. Bit line pair BLP includes bit linesBL and /BL through which complementary signals are transmitted, andinputs and outputs a data signal to and from a selected memory cell MC.

Bit line pair BLP in an odd column of memory block BK1 is connected viaa transfer gate 31 to sense amplifier 22, and is further connected via atransfer gate 34 to bit line pair BLP in an odd column of memory blockBK2. Transfer gate 31 includes N channel MOS transistors 32 and 33 thatare respectively connected between bit lines BL, /BL and input/outputnodes N1, N2 of sense amplifier 22. N channel MOS transistors 32 and 33have their gates receiving a signal BLIL1. Transfer gate 34 includes Nchannel MOS transistors 35 and 36 that are respectively connectedbetween bit lines BL, /BL and input/output nodes N1, N2 of senseamplifier 22. N channel MOS transistors 35 and 36 have their gatesreceiving a signal BLIR1. Transfer gates 31 and 34 connect a selectedmemory block (BK1, for example) out of memory blocks BK1 and BK2 tosense amplifier 22, and disconnect the other memory block (BK2, in thiscase) from sense amplifier 22.

Sense amplifier 22 includes N channel MOS transistors 23 and 24connected between input/output nodes N1, N2 and a node N3, respectively,and P channel MOS transistors 26 and 27 connected between input/outputnodes N1, N2 and a node N4, respectively. MOS transistors 23 and 26 havetheir gates connected to input/output node N2, and MOS transistors 24and 27 have their gates connected to input/output node N1. Further,sense amplifier 22 includes an N channel MOS transistor 25 connectedbetween node N3 and a node of ground potential GND, and a P channel MOStransistor 28 connected between node N4 and a node of power supplypotential VCC. MOS transistors 25 and 28 have their gates receivingsense amplifier activation signals SANE and SAPE, respectively. Senseamplifier 22 amplifies a small potential difference produced between bitlines BL and /BL after activation of a memory cell MC, to power supplyvoltage VCC.

Furthermore, a bit line equalize circuit 40 for equalizing bit lines BL,/BL to a bit line precharge potential VCC/2 before activation of memorycell MC is provided between transfer gates 31 and 34. Bit line equalizecircuit 40 includes N channel MOS transistors 41 and 42 connectedbetween input/output nodes N1, N2 and a node N5, respectively, and an Nchannel MOS transistor 43 connected between input/output nodes N1 andN2. MOS transistors 41-43 have their gates receiving a bit line equalizesignal BLEQ. Bit line precharge potential VCC/2 is applied to node N5.

This complete hidden refresh function-equipped DRAM employs a 2-waymethod as well. A plurality of word lines WLs in memory block BK1 aredivided into two ways W0 and W1. Way W0 includes a word line WL in anodd row, and way W1 includes a word line WL in an even row. SignalsRX0-1 and RX1-1 are assigned to Ways W0 and W1, respectively, of memoryblock BK1, and signal group X is assigned to respective word lines WLsbelonging to ways W0 and W1. Respective word lines WLs of memory blockBK1 are specified by signals RX0-1 and RX1-1 and signal group X.

In order to realize this 2-way method, word driver group WD1 includes aword driver (AND gate) 50 provided corresponding to each odd row ofmemory block BK1, a word driver (AND gate) 51 provided corresponding toeach even row thereof, and a word driver (AND gate) 52 providedcorresponding to each pair of adjacent word drivers 50 and 51. Worddriver 52 receives a signal group X1. Word driver 50 receives an outputof word driver 52 and signal RX0-1. Word driver 51 receives an output ofword driver 52 and signal RX1-1. Outputs of word drivers 50 and 51 arerespectively applied to corresponding word lines WLs. For example, ifall signals of signal group X1 attain an active level of an “H” level(boosted potential Vpp) and signal RX0-1 for selecting way W0 attains anactive level of an “H” level (boosted potential Vpp), the first wordline WL1 is set to a selected level of an “H” level (boosted potentialVpp). The same applies to other memory blocks BK2-BKm. It is noted thatword drivers 50-52 may be formed of CMOS transistors or N channel MOStransistors. Word drivers 50-52 are driven by boosted potential Vpp andground potential GND.

Hereinafter, an operation of the complete hidden refreshfunction-equipped DRAM shown in FIGS. 1-3 will be described briefly. Inthe write mode, bit line pair BLP in the column corresponding to columnaddress signals A0-Ai is selected by column decoder 13. Selected bitline pair BLP is connected to input buffers 18 and 20 via senseamplifier+input/output controller circuit 16 and global signalinput/output line pair group GIO. Input buffers 18 and 20, in responseto signal /WE, apply the write data signals from data input/outputterminal groups 4 and 5 to the selected bit line pair BLP via globalsignal input/output line pair group GIO. The write data is provided as apotential difference between bit lines BL and /BL. Thereafter, rowdecoder 14 causes word line WL in the row corresponding to row addresssignals Ai+1 to Aj to rise to a selected level of an “H” level for aprescribed time, to render the MOS transistor Q of memory cell MC in therelevant row conductive. Charges corresponding to the potential of bitline BL or /BL are stored in capacitor C of the selected memory cell MC.

Charges in capacitor C of memory cell MC flow out gradually, so thatdata refresh is carried out. In the case where the internal row addresssignal generated in row decoder 14 is a signal for designating a wordline WL in memory block BK1, in FIG. 3, signals BLIR1 and BLEQ fall froman “H” level to an “L” level, and MOS transistors 35 and 36 of transfergate 34 and MOS transistors 41-43 of bit line equalize circuit 40 arerendered nonconductive. Row decoder 14 causes word line WL in the rowcorresponding to the relevant internal address signal to rise to an “H”level. The potentials of bit lines BL and /BL change slightly accordingto the amount of charges in capacitor C of the activated memory cell MC.

Next, sense amplifier activation signal SANE is pulled up to an “H”level, and sense amplifier activation signal SAPE is pulled down to an“L” level, whereby sense amplifier 22 is activated. If the potential ofbit line BL is slightly higher than that of bit line /BL, the resistancevalues of MOS transistors 24 and 26 are lower than the resistance valuesof MOS transistors 23 and 27, and the potential of bit line BL is pulledup to an “H” level, and the potential of bit line /BL is pulled down toan “L” level. Conversely, if the potential of bit line /BL is slightlyhigher than that of bit line BL, the resistance values of MOStransistors 23 and 27 are smaller than the resistance values of MOStransistors 24 and 26. The potential of bit line /BL is pulled up to an“H” level, and the potential of bit line BL is pulled down to an “L”level. Word line WL is pulled down to a non-selected level of an “L”level, signals BLIR1, BLEQ, SANE and SAPE are reset, and thus, datarefresh (rewrite) for the relevant word line WL is completed. Theabove-described cycle is carried out for each word line WL of memoryblock BK1, and then carried out for each word line WL of memory blockBK2.

In the read mode, data of memory cells MCs in the row selected by rowdecoder 14 are read onto bit line pairs BLPs in a similar manner as inthe refresh mode, and data in bit line pair BLP in the column selectedby column decoder 13 is provided to output buffers 19 and 21 via globalsignal input/output line pair group GIO. Output buffers 19 and 21 outputread data Q to data input/output terminal groups 4 and 5 in response tosignal /OE.

Hereinafter, the refresh method that is a feature of this completehidden refresh function-equipped DRAM will be explained in detail. FIG.4 is a circuit block diagram showing a configuration of a refreshinstruction circuit 55 that is included in control circuit 10 and thatoutputs a refresh instruction signal /REFE. Referring to FIG. 4, refreshinstruction circuit 55 includes an instruction signal activating circuit56, a determination circuit 57, NAND gates 58 and 61, an inverter 59,delay circuits 60 and 66, a flip-flop 62, and a buffer 65.

Instruction signal activating circuit 56 sets a refresh flag signalRefflag to an active level of an “H” level periodically, to activaterefresh instruction signal /REFE. Determination circuit 57 determineswhether to output refresh instruction signal /REFE or not, and sets adetermination signal Refwin to an active level of an “H” level when thesignal /REFE is to be output, and sets determination signal Refwin to aninactive level of an “L” level when the signal /REFE is not to beoutput.

NAND gate 58 receives refresh flag signal Refflag and determinationsignal Refwin, and outputs an inverted signal /REFSF of their logicalproduct signal. Signal /REFSF attains an active level of an “L” levelwhen refresh flag signal Refflag and determination signal Refwin areboth at an “H” level. Inverter 59 outputs an inverted signal φA1 ofoutput signal /REFSF of NAND gate 58. Delay circuit 60 delays signal/REFSF by a prescribed period of time. NAND gate 61 receives outputsignal φA1 of inverter 59 and an output signal of delay circuit 60, andoutputs an inverted signal /REFS of their logical product signal. Signal/REFS is set to an “L” level for a period of delay by delay circuit 60,in response to falling of signal /REFSF from an “H” level to an “L”level.

Flip-flop 62 is formed of NAND gates 63 and 64. NAND gate 63 receivessignal /REFS and an output signal φA3 of NAND gate 64, and outputs aninverted signal φA2 of their logical product signal. NAND gate 65receives output signal φA2 of NAND gate 64 and an output signal φA4 ofdelay circuit 66, and outputs an inverted signal φA3 of their logicalproduct signal. Flip-flop 62 is set in response to a falling edge ofsignal /REFS, and reset in response to a falling edge of signal φA4.When flip-flop 62 is set, signal φA3 attains an “L” level. Whenflip-flop 62 is reset, signal φA3 attains an “H” level. Buffer 65buffers signal φA3, and outputs it as refresh instruction signal /REFE.Delay circuit 66 outputs signal φA4 that corresponds to refreshinstruction signal /REFE delayed by a prescribed period of time.

FIG. 5 is a circuit block diagram showing a configuration of instructionsignal activating circuit 56 in FIG. 4. Referring to FIG. 5, instructionsignal activating circuit 56 includes a timer circuit 71, a flip-flop72, a NAND gate 75, inverters 76 and 77, and a delay circuit 78. Timercircuit 71 includes a ring oscillator, and outputs a cycle signal/Refcyc that is set to an active level of an “L” level at a specifiedperiod. Inverter 77 outputs an inverted signal of refresh instructionsignal /REFE. Delay circuit 78 outputs a signal φA13 that corresponds tothe output signal of inverter 77 delayed by a prescribed period of time.NAND gate 75 receives refresh instruction signal /REFE and output signalφA13 of delay circuit 78, and outputs an inverted signal φA12 of theirlogical product signal. Signal φA12 is set to an “L” level for a periodof delay of delay circuit 78 in response to rising of signal /REFE froman “L” level to an “H” level.

Flip-flop 72 is formed of NAND gates 73 and 74. NAND gate 73 receivescycle signal /Refcyc and an output signal φA11 of NAND gate 74, andoutputs an inverted signal φA10 of their logical product signal. NANDgate 74 receives output signal φA10 of NAND gate 73 and output signalφA12 of NAND gate 75, and outputs an inverted signal φA11 of theirlogical product signal. Flip-flop 72 is set in response to a fallingedge of signal /Refcyc, and reset in response to a falling edge ofsignal φA12. When flip-flop 72 is set, signal φA11 attains an “L” level.When flip-flop 72 is reset, signal φA11 attains an “H” level. Inverter76 outputs an inverted signal of output signal φA11 of flip-flop 72 asrefresh flag signal Refflag.

FIG. 6 is a circuit block diagram showing a configuration ofdetermination circuit 57 in FIG. 4. Referring to FIG. 6, determinationcircuit 57 includes AND gates 81 and 82, an OR gate 83, an inverter 84,and a delay circuit 85. AND gate 81 receives an internal read enablesignal int/RE and an internal write enable signal int/WE, and outputstheir logical product signal φA20. It is noted that control circuit 10generates internal read enable signal int/RE in response to outputenable signal /OE externally provided via control signal input terminalgroup 1, and generates internal write enable signal int/WE in responseto write enable signal /WE externally provided via control signal inputterminal group 1.

Inverter 84 outputs an inverted signal of output signal φA20 of AND gate81. Delay circuit 85 outputs a signal φA21 that corresponds to theoutput signal of inverter 84 delayed by a prescribed period of time. ANDgate 82 receives output signal φA20 of AND gate 81 and output signalφA21 of delay circuit 85, and outputs their logical product signal φA22.OR gate 83 receives output signal φA22 of AND gate 82 and an internalchip enable signal int/CE, and outputs their logical sum signal asdetermination signal Refwin. It is noted that control circuit 10generates internal chip enable signal int/CE in response to chip enablesignal /CE externally provided via control signal input terminal group1.

Hereinafter, an operation of refresh instruction circuit 55 shown inFIGS. 4-6 will be described. Firstly, an operation of refreshinstruction circuit 55 in the case where data write and data read arenot performed, i.e., in the case where internal read enable signalint/RE and internal write enable signal int/WE are both fixed at an “H”level, will be described with reference to FIG. 7.

When internal read enable signal int/RE and internal write enable signalint/WE are both fixed at an “H” level, output signal φA22 of AND gate 82in FIG. 6 is fixed at an “L” level, and internal chip enable signalint/CE, i.e., chip enable signal /CE, becomes determination signalRefwin. More specifically, when chip enable signal /CE is at an inactivelevel of an “H” level, the complete hidden refresh function-equippedDRAM is set to a standby mode. Accordingly, determination circuit 57determines that the refresh operation can be carried out, and thatrefresh instruction circuit 55 can set refresh instruction signal /REFEto an active level of an “L” level, and thus, it sets determinationsignal Refwin to an active level of an “H” level.

At time t1, when cycle signal /Refcyc output from timer circuit 71 islowered from an “H” level to an “L” level, flip-flop 72 is set, andrefresh flag signal Refflag is raised from an “L” level to an “H” level.In response thereto, NAND gate 58 in FIG. 4 receives determinationsignal Refwin set to an “H” level and refresh flag signal Refflag set toan “H” level, and sets signal /REFSF to an active level of an “L” level.NAND gate 61 sets signal /REFS to an “L” level for a time period thatcorresponds to the delay time of delay circuit 60.

Flip-flop 62 is set in response to a falling edge of signal /REFS, andrefresh instruction signal /REFE is pulled down to an active level of an“L” level. After a lapse of the delay time of delay circuit 66,flip-flop 62 is reset, and refresh instruction signal /REFE is raised toan “H” level. In this manner, refresh instruction signal /REFE is set toan active level of an “L” level for a prescribed period of time fromtime t1.

As described above, when refresh flag signal Refflag output frominstruction signal activating circuit 56 is activated at time t1,determination circuit 57 determines that the refresh operation can becarried out, and sets determination signal Refwin to an active level ofan “H” level. This allows the refresh operation to be carried out whilethe complete hidden refresh function-equipped DRAM is in the standbymode.

Refresh instruction signal /REFE output from refresh instruction circuit55 is inactivated at time t2 after a lapse of a prescribed time set bydelay circuit 66. At this time, output signal φA12 of NAND gate 75 inFIG. 5 attains an “L” level, flip-flop 72 is reset, and refresh flagsignal Refflag is set to an inactive level of an “L” level.

Next, at time t2′, chip enable signal /CE is pulled down to an activelevel of an “L” level. The complete hidden refresh function-equippedDRAM is set to an active mode, and determination circuit 57 determinesthat the refresh operation cannot be carried out unless the read orwrite operation is finished, and thus, it sets determination signalRefwin to an inactive level of an “L” level.

Next, at time t3, cycle signal /Refcyc, which is activated at aspecified period, is set to an active level of an “L” level.Correspondingly, flip-flop 72 in FIG. 5 is set, and refresh flag signalRefflag is set to an active level of an “H” level.

Determination signal Refwin output from determination circuit 57 howeveris at an inactive level of an “L” level, and thus, output signal /REFSof NAND gate 61 maintains an inactive level of an “H” level.Accordingly, refresh instruction signal /REFE remains at an inactivelevel of an “H” level.

Since refresh instruction signal /REFE maintains an “H” level, outputsignal φA12 of NAND gate 75 in FIG. 5 remains at an “H” level, andoutput signal φA11 of flip-flop 72 remains at an “L” level. As a result,refresh flag signal Refflag is at an active level of an “H” level aftertime t3.

As described above, during the time when chip enable signal /CE is at anactive level of an “L” level, determination circuit 57 determines thatthe refresh operation cannot be performed unless the read or writeoperation is finished. When refresh flag signal Refflag is set to anactive level of an “H” level during the time when determination circuit57 determines that the refresh operation cannot be performed, refreshflag signal Refflag is maintained at an “H” level.

Next, at time t4, if chip enable signal /CE is set to an inactive levelof an “H” level and the complete hidden refresh function-equipped DRAMis set again to a standby mode without performing the read or writeoperation, determination circuit 57 determines that the refreshoperation can be carried out, and thus rises determination signal Refwinto an active level of an “H” level.

Here, since refresh flag signal Refflag is in an active state after timet3, at time t4, output signal /REFS of NAND gate 61 in FIG. 4 attains anactive level of an “L” level only for a prescribed period of time set bydelay circuit 60. In response, flip-flop 62 is set, and refreshinstruction signal /REFE is set to an active level of an “L” level onlyfor a prescribed period of time set by delay circuit 66.

At time t5, i.e., after a lapse of the prescribed period of time set bydelay circuit 66 from time t4, refresh instruction signal /REFE isinactivated. In response to inactivation of refresh instruction signal/REFE, refresh flag signal Refflag is also inactivated.

After time t5 as well, refresh instruction circuit 55 activates refreshinstruction signal /REFE in response to refresh flag signal Refflagevery time refresh flag signal Refflag is activated during the period inwhich determination circuit 57 determines that the refresh operation canbe carried out.

By the operation described above, determination circuit 57 determinesthat the refresh operation can be carried out when chip enable signal/CE is at an inactive level of an “H” level, i.e., when the completehidden refresh function-equipped DRAM is in a standby mode. When chipenable signal /CE is at an active level of an “L” level, i.e., when thecomplete hidden refresh function-equipped DRAM is in an active mode,determination circuit 57 determines that the refresh operation cannot beperformed until the read or write operation is finished.

Further, during the time in which determination circuit 57 determinesthat the refresh operation cannot be carried out, if cycle signal/Refcyc output from timer circuit 71 is activated, refresh flag signalRefflag is maintained in an active state. That is, the refresh operationis awaited. This enables the refresh operation to be carried outimmediately once determination circuit 57 determines that it is possibleto carry out the refresh operation.

FIG. 8 is a timing chart illustrating an operation of refreshinstruction circuit 55 in the case where data write and/or read iscarried out, i.e., in the case where internal read enable signal int/REand/or internal write enable signal int/WE is set to an active level ofan “L” level.

Referring to FIG. 8, at time t 1, when chip enable signal /CE is set toan active level of an “L” level, the complete hidden refreshfunction-equipped DRAM is set to an active mode. Since internal readenable signal int/RE or internal write enable signal int/WE is at aninactive level of an “H” level at time t1, output signal φA22 of ANDgate 82 in FIG. 6 is at an “L” level. Thus, determination signal Refwinoutput from OR gate 83 attains an inactive level of an “L” level.

When internal read enable signal int/RE or internal write enable signalint/WE is set to an active level of an “L” level at time t2, the read orwrite operation is initiated. At this time, output signal φA20 of ANDgate 81 in FIG. 6 attains an “L” level, and output signal φA22 of ANDgate 82 attains an “L” level. Accordingly, determination signal Refwinoutput from OR gate 83 is maintained at an inactive level of an “L”level.

At time t3, internal read enable signal int/RE or internal write enablesignal int/W is raised to an inactive level of an “H” level, and theread or write operation is finished. At this time, output signal φA20 ofAND gate 81 in FIG. 6 attains an “H” level. Output signal φA21 of delaycircuit 85 is maintained at an “H” level for a specified time Δt fromtime t3. Thus, output signal φA22 of AND gate 82 is at an “H” level forspecified period Δt from time t3, and accordingly, determination signalRefwin output from OR gate 83 is maintained at an active level of an “H”level for specified period Δt from time t3. By the operation describedabove, determination circuit 57 determines that the refresh operationcan be carried out during the specified time Δt after completion of theread or write operation.

At time t3, although determination signal Refwin is activated, cyclesignal /Refcyc is still in an inactive state. Thus, refresh flag signalRefflag output from instruction signal activating circuit 56 is in aninactive state as well. Thus, at time t3, refresh instruction signal/REFE output from refresh instruction circuit 55 remains at an inactivelevel of an “H” level.

When cycle signal /Refcyc is pulled down to an active level of an “L”level at time t4, refresh flag signal Refflag is raised to an activelevel of an “H” level. At this time, determination signal Refwin is atan inactive level of an “L” level. Thus, refresh instruction signal/REFE is maintained at an inactive level of an “H” level. After time t4,refresh flag signal Refflag is maintained at an active level of an “H”level.

At time t5, the read or write operation is initiated again. When theread or write operation is finished at time t6, determination signalRefwin is set to an active state for a specified time Δt from time t6,as in the case of time t4.

Refresh flag signal Refflag is maintained in an active state since timet4. Thus, output signal /REFS of NAND gate 61 in FIG. 4 attains anactive level of an “L” level for a prescribed period of time set bydelay circuit 60. When signal /REFS is pulled down to an “L” level,refresh instruction signal /REFE attains an active level of an “L” levelwithin a prescribed period of time set by delay circuit 66 after timet6. The refresh operation is thus carried out. When refresh instructionsignal /REFE is inactivated at time t7, refresh flag signal Refflag isinactivated correspondingly.

If internal read enable signal int/RE or internal write enable signalint/WE is set to an active level of an “L” level again during therefresh operation from time t6 to time t7, the read or write operationis carried out after completion of the refresh during time t6 to t7.

With the operation described above, determination circuit 57 determinesthat the refresh operation can be performed during a prescribed periodof time after completion of the read or write operation even in anactive mode. Thus, when determination circuit 57 determines that therefresh operation can be carried out, if refresh flag signal Refflag isin an active state, refresh instruction signal /REFE is activated. Thatis, the complete hidden refresh function-equipped DRAM performs therefresh operation after completion of the write or read operation. Thismeans that the refresh operation would not be carried out at the sametiming as the read or write operation even if the complete hiddenrefresh function-equipped DRAM is in the active mode. Further, even inthe case where chip enable signal /CE is in an active level for a longperiod of time, data destruction due to the absence of the refreshoperation for a long time can be prevented. Furthermore, rapid access ispossible since the refresh operation is carried out after the write orread operation without fail.

Still further, the complete hidden refresh function-equipped DRAM doesnot reset each of way selection signals RX0-1, RX1-1, . . . to an “L”level at each cycle in the case where the refresh operation is carriedout in a standby mode, for the purpose of decreasing consumed power.Hereinafter, the operation will be described in detail.

FIG. 9 is a circuit block diagram showing a portion of row decoder 14related to data refresh. Referring to FIG. 9, row decoder 14 includes anaddress generating circuit 90, a refresh start trigger generatingcircuit 91, a refresh address change sensing circuit 92, AND gates 93and 100, a flip-flop 94, a NAND gate 97, and latch circuits 98 and 99.

Address generating circuit 90 includes an oscillator 101 and an addresscounter 102, as shown in FIG. 10. Oscillator 101 is activated whenrefresh instruction signal /REFE output from control circuit 10 is at an“L” level, and outputs an internal clock signal int/RAS of a prescribedfrequency. Address counter 102 includes a plurality of flip-flopsFF0-FFq connected in series, and counts the number of pulses of internalclock signal int/RAS output from oscillator 101. The outputs offlip-flops FF0-FFq become refreshing row address signals C0-Cq,respectively. Row address signals C0-C6 correspond to signal group X.Row address signal C7 is used for selecting a way. Row address signalsC8-Cq are used for selecting a memory block BK.

Refresh start trigger generating circuit 91 normally outputs a signal ofan “H” level, and outputs a pulse of an “L” level in response to refreshinstruction signal /REFE attaining an active level of an “L” level.Refresh address change sensing circuit 92 normally outputs a signal ofan “H” level, and outputs a pulse of an “L” level in response to achange in address signal C7, that is, an output of flip-flop FF7 inaddress generating circuit 90. AND gate 93 receives an output signal ofrefresh start trigger generating circuit 91 and an output signal ofrefresh address change sensing circuit 92, and outputs their logicalproduct signal /RATD.

Flip-flop 94 includes two NAND gates 95 and 96. Flip-flop 94 is set inaccordance with a falling edge of signal /RATD, and reset in accordancewith a falling edge of internal clock signal int/RAS output fromoscillator 101 in address generating circuit 90. NAND gate 97 receivesan output signal of flip-flop 94 and internal chip enable signal int/CE,and outputs an inverted signal /HOLD of their logical product signal.

Latch circuit 98 includes a transfer gate 111 and inverters 112-114, asshown in FIG. 11. Transfer gate 111 is connected between an input nodeN10 and an intermediate node N11, inverter 112 is connected betweenintermediate node N11 and an output node N12, and inverter 113 isconnected between output node N12 and intermediate node N11. Signal/HOLD is directly input to a gate 111 a of transfer gate 11 on the sideof an N channel MOS transistor, and also input via inverter 114 to agate 111 b of transfer gate 111 on the side of a P channel MOStransistor. Accordingly, an input level at the time when signal /HOLDfalls from an “H” level to an “L” level is latched by inverters 112 and113. Latch circuit 99 has a structure similar to that of latch circuit98, to which an inverter 115 is added, as shown in FIG. 12. Inverter 115is connected between an output node of inverter 112 and output node N12.A signal RXM is input to latch circuit 98, while a signal φBL0-1 isinput to latch circuit 99.

AND gate 100 receives output signals Pre.RX and Pre.BS0-1 of latchcircuits 98 and 99, respectively. An output of AND gate 100 is a signalRX0-1. Latch circuit 99 and AND gate 100 are provided corresponding toeach of signals RX0-1, RX1-1 to RX0-m, RX1-m.

Hereinafter, an operation of the circuit portion shown in FIGS. 9-12will be described. FIG. 13 is a timing chart illustrating the operationof row decoder 14 in the case where internal chip enable signal int/CEis at an “H” level, i.e., in a standby mode. In this case, NAND gate 97operates as an inverter with respect to an output signal of flip-flop94. When refresh instruction signal /REFE is pulled down to an “L” levelof an active level, internal clock signal int/RAS is output fromoscillator 101 of address generating circuit 90, and a countingoperation of address counter 102 is initiated.

A pulse signal P1 at an “L” level is output from refresh start triggergenerating circuit 91 in response to refresh instruction signal /REFEattaining an “L” level. Pulse signals P2, P3, . . . at an “L” level areoutput from refresh address change sensing circuit 92 in response to achange in address signal C7, i.e., an output signal of flip-flop FF7 ofaddress generating circuit 90. Pulse signals P1, P2, P3, . . . passthrough AND gate 93 to be a signal /RATD.

Flip-flop 94 is set in accordance with a falling edge of signal /RATD toan “L” level, and is reset in accordance with a falling edge of internalclock signal int/RAS to an “L” level. When flip-flop 94 is set, theoutput signal of flip-flop 94 is pulled down to an “L” level. Whenflip-flop 94 is reset, the output signal of flip-flop 94 is raised to an“H” level. Here, since internal chip enable signal int/CE is at an “H”level, the inverted signal of the output signal of flip-flop 94 becomessignal /HOLD.

Signals φBL0-1 and φBL1-1 are signals generated within row decoder 14based on internal clock signal int/RAS and the outputs C7-Cq offlip-flops FF7-FFq of address generating circuit 90. Signal φBL0-1indicates that one way W0 of memory block BK1 has been selected, whichis an inverted signal of internal clock signal int/RAS during the periodin which way W0 of memory block BK1 is selected. Signal φBL1-1 indicatesthat other way W1 of memory block BK1 has been selected, which is aninverted signal of internal clock signal int/RAS during the period inwhich way W1 of memory block BK1 is selected.

Signal φBL0-1 is latched by latch circuit 99 when signal /HOLD fallsfrom an “H” level to an “L” level, and is unlatched from latch circuit99 when signal /HOLD rises from an “L” level to an “H” level. An outputof latch circuit 99 becomes a signal Pre.BS0-1. Similarly, signal φBL1-1becomes a signal Pre.BS1-1. As such, for each of signals φBL0-1 andφBL1-1, the portion that swings at the same period as that of internalclock signal int/RAS is made constant at an “H” level.

Signal RXM is a signal that swings at approximately the same timing asinternal clock signal int/RAS, and is output from control circuit 10.Signal RXM is latched by latch circuit 98 when signal /HOLD falls froman “H” level to an “L” level, and is unlatched from latch circuit 98when signal /HOLD rises from an “L” level to an “H” level. An output oflatch circuit 98 is a signal Pre.RX. Thus, signal Pre.RX is an invertedsignal of signal /HOLD. A logical product signal of signals Pre.RX andPre.BS0-1 is a signal RX0-1. A logical product signal of signals Pre.RXand Pre.BS1-1 is a signal RX1-1. Word drivers 50 and 51 of FIG. 3 areactivated by these signals RX0-1 and RX1-1.

While word driver 50 of way W0 is activated by signal RX0-1, word linesWLs belonging to way W0 are sequentially selected, so that data refreshis carried out. While word driver 51 of way W1 is activated by signalRX1-1, word lines WLs belonging to way W1 are sequentially selected, sothat data refresh is carried out. Then, a block BK2 is selected, and asimilar operation is carried out.

When internal chip enable signal int/CE is at an “L” level, i.e., in anactive mode, signal /HOLD is fixed at an “H” level, and transfer gate111 of each of latch circuits 98 and 99 is rendered conductive. Thus,the inverted signal of signal RXM becomes signal Pre.RX, and signalφBL0-1 becomes signal Pre.BS0-1. As such, signals RX0-1, RX1-1 becomesignals φBL0-1 and φBL1-1, respectively, which swing at each cycle.

In a standby mode, an address to be selected in a next cycle can beexpected, without interruption of a read or write operation during therefresh operation. Thus, it is no problem to maintain the way selectsignal RX at an “H” level, instead of resetting the same, even after therefresh cycle is finished. In the active mode, however, the refreshoperation is carried out by interrupting the read or write operation.Thus, it is not possible to expect which address is to be selected afterthe refresh cycle. Therefore, it is necessary to reset way select signalRX for each refresh cycle so as to enable rapid access in a cyclefollowing the refresh cycle.

In the first embodiment, when chip enable signal /CE is at an “H” level,i.e., in the standby mode, an upper address is assigned to each of waysW0 and W1 and a lower address is assigned to each word line WL belongingto each of ways W0 and W1. Furthermore, while a word line WL in one wayW (W0, for example) of one memory block BK (BK1, for example) isselected, a signal RX (RX0-1, in this case) is retained at an “H” levelof an active level (boosted voltage Vpp) without being reset.Consequently, power consumption is reduced compared to the conventionalexample in which a signal RX is raised to an “H” level and then reset toan “L” level every time one word line WL is selected. More specifically,since signal RX needs to be reset only once for a period during whichn/2 word lines WLs are selected (only once for k word lines (wherek<n/2) in the case of the initially selected way W0), power required toreset signal RX is reduced to about 2/n compared to the conventionalexample. Since the number of word lines WLs per one memory block BK isnormally 256 or 512, power consumption will be reduced to one-severalhundredth. Moreover, since voltage Vpp of the amplitude of signal RX isgreater than power supply voltage VCC, the effect of reducing the powerconsumption by decreasing the number of times of reset of signal RX issignificant.

Further, when chip enable signal /CE is at an “L” level, i.e., in theactive mode, signal RX is reset at each cycle, so that the write andread operations can be carried out quickly.

Although the number of ways has been set to two in the first embodiment,the same effect can of course be obtained with three ways or more.

Second Embodiment

FIG. 14 is a diagram showing layout of a row decoder 14 and a memory mat17 of a complete hidden refresh function-equipped DRAM according to asecond embodiment of the present invention, and FIG. 15 is an enlargedview of a main part of FIG. 14.

Referring to FIGS. 14 and 15, a divided word line method and a 2-waymethod are used in this complete hidden refresh function-equipped DRAM.Each word line WL in each of memory blocks BK1-BKm is divided into aplurality of sub-word lines SWLs, each of memory blocks BK1-BKm isdivided into a plurality of sub-blocks 120, and a SD band 121 isprovided corresponding to each sub-block 120.

A plurality of sub-word lines SWLs of each sub-block 120 are dividedinto two ways W0 and W1. Way W0 includes a sub-word line SWL in an oddrow, and way W1 includes a sub-word line SWL in an even row. Sub-decodesignals SD0 and SD1 are assigned to ways W0 and W1, respectively, and asignal group X is assigned to each sub-word line SWL belonging to eachof ways W0 and W1. Respective sub-word lines SWLs of each sub-block 120are specified by signals SD0 and SD1 and signal group X.

In order to realize the 2-way method, each SD band includes a worddriver 122 provided corresponding to each odd row of correspondingsub-block 120, and a word driver 123 provided corresponding to each evenrow of corresponding sub-block 120. Further, each of word driver groupsWD1-WDm includes a word driver 52 provided corresponding to adjacentword drivers 122 and 123 of SD band 121 of corresponding one of memoryblocks BK1-BKm. Word driver 52 receives signal group X. Word driver 122receives an output of word driver 52 and signal SD0. Word driver 123receives the output of word driver 52 and signal SD1 Outputs of worddrivers 122 and 123 are applied respectively to corresponding sub-wordlines SWLs. Each of word drivers 52, 122 and 123 is driven by boostedpotential Vpp and ground potential GND. The selected level of the mainword line MWL, the selected level of sub-word line SWL, the activelevels of signals SD0 and SD1, and the active level of signal group Xall correspond to boosted potential Vpp.

FIG. 16 is a circuit block diagram showing a circuit for generating asignal SD in the complete hidden refresh function-equipped DRAM shown inFIGS. 14 and 15, and FIG. 17 is a timing chart illustrating an operationthereof.

The circuit configuration and operation shown in FIGS. 16 and 17 aresimilar to those of FIGS. 9 and 13, except that signals RXM, Pre.RX,RX0-1 and RX1-1 are replaced with signals SDM, Pre.SD, SD0 and SD1,respectively. More specifically, in the case where chip enable signal/CE is at an “H” level, signals SD0 and SD1 for selecting ways W0 and W1are not reset while sub-word lines SWLs belonging to respective ways W0and W1 are selected. In the case where chip enable signal /CE is at an“L” level, signals SD0 and SD1 for selecting ways W0 and W1 are reset ateach cycle. The other configuration and operation are identical to thoseof the first embodiment, and thus, description thereof will not berepeated here.

The same effects as those of the first embodiment can be obtained in thesecond embodiment.

Third Embodiment

FIG. 18 is a diagram showing layout of a row decoder 14 and a memory mat17 of a complete hidden refresh function-equipped DRAM according to athird embodiment of the present invention, and FIG. 19 is a circuitdiagram showing a main part of a word driver group WD shown in FIG. 18.

Referring to FIGS. 18 and 19, in this complete hidden refreshfunction-equipped DRAM, predecode signal groups XJ, XK, XL and Reset areformed in row decoder 14 in place of signal group X. Predecode signalgroups XJ, XK, XL and Reset are applied to each word driver 52. Each ofword drivers 50-52 is driven by boosted potential Vpp and groundpotential GND. The selected level of word line MWL, the active levels ofsignals RX0-1 and RX1-1, and the active levels of signals XJ, XK, XL andReset all correspond to boosted potential Vpp.

FIG. 20 is a circuit block diagram showing a portion of row decoder 14related to the refresh operation. FIG. 21 is a circuit block diagramshowing a configuration of an address generating circuit 90.

Referring to FIGS. 20 and 21, this complete hidden refreshfunction-equipped DRAM differs from the complete hidden refreshfunction-equipped DRAM of the first embodiment in that latch circuit 99and AND gate 100 are removed, address signals C8-Cq relating toselection of block BK are output from flip-flops FF0-FF6 of addressgenerating circuit 90, address signals C0-C6 relating to predecodesignals XJ, XK and XL are output from flip-flops FF7 to FFq-1, and asignal C7 relating to selection of a way W is output from flip-flop FFq.Refresh address change sensing circuit 92 outputs a pulse signal inresponse to a change in address signal C0, i.e., an output of flip-flopFF7 of address generating circuit 90. A signal XJM is input to latchcircuit 98, and an output signal of latch circuit 98 becomes signal XJ.Latch circuit 98 is provided corresponding to each of predecode signalsXJ, XK, XL and Reset.

FIG. 22 is a timing chart illustrating an operation of the completehidden refresh function-equipped DRAM shown in FIGS. 18-21. Signal /HOLDis generated in a similar manner as in the first embodiment. Signal XJMis latched by latch circuit 98 when signal /HOLD falls from an “H” levelto an “L” level, and unlatched from latch circuit 98 when signal /HOLDrises from an “L” level to an “H” level. An output of latch circuit 98becomes signal XJ. The same applies to other signals XK, XL and Reset.

While two word drivers 50 and 51 in each of blocks BK1-BKm are activatedby predecode signals XJ, XK, XL and Reset, signals RX0-1 to RX0-m orsignals RX1-1 to RX1-m attain an “H” level sequentially, andcorresponding word lines WLs in respective blocks BK1 to BKm areselected sequentially, so that data refresh is carried out. At the timeof start of refresh, while two word drivers 50 and 51 in each of blocksBK1 to BKm are activated by predecode signals XJ, XK, XL and Reset,signals RX0-h (h>1) to RX0-m or signals RXh-1 to RX1-m attain an “H”level sequentially, and corresponding word lines WLs in respectiveblocks BKh to BKm are sequentially selected, so that data refresh iscarried out. The other configuration and operation are identical tothose of the first embodiment, and thus, description thereof will not berepeated here.

In the third embodiment, in the case where chip enable signal /CE is atan “H” level, i.e., in a standby mode, a lower address is assigned toeach of memory blocks BK1 to BKm, and an upper address is assigned toeach word line WL belonging to each of memory blocks BK1 to BKm, andfurther, predecode signals XJ, XK, XL and Reset will not be reset whilecorresponding word lines WLs in respective blocks BK1 to BKm areselected. Consequently, power consumption can be reduced compared to aconventional example in which predecode signals XJ, XK, XL and Reset arereset every time a single word line WL is selected.

Further, in the case where chip enable signal /CE is at an “L” level,i.e., in an active mode, predecode signals XJ, XK, XL and Reset arereset at each cycle, so that the write and read operations can becarried out quickly.

Fourth Embodiment

FIG. 23 is a circuit block diagram showing a configuration of a mainpart of a complete hidden refresh function-equipped DRAM according to afourth embodiment of the present invention.

Referring to FIG. 23, this complete hidden refresh function-equippedDRAM differs from the complete hidden refresh function-equipped DRAM ofthe first embodiment in that signals RXM, Pre.RX, φBL0-1, Pre.BS0-1, andRX0-1 are replaced with signals BLIM, Pre.BL1, φBL1, Pre.BS1, and BLIR1,respectively, and AND gate 100 is replaced with a NAND gate 125.Further, refresh address change sensing circuit 92 normally outputs asignal of an “H” level, and outputs a pulse of an “L” level in responseto a change in an address signal C8, i.e., an output of flip-flop FF8 ofaddress generating circuit 90. Signal BLIM is input to latch circuit 98,and a block selection signal φBL1 is input to latch circuit 99. NANDgate 125 receives an output signal Pre.BLI of latch circuit 98 and anoutput signal Pre.BS1 of latch circuit 99, and outputs a signal BLIR1.Latch circuit 99 and NAND gate 125 are provided corresponding to each ofsignals BLIL1, BLIR1, BLIL2, BLIR2, . . . . Signals φBL2, φBL1, φBL3,φBL2, . . . for selecting a block to which corresponding signals BLIL1,BLIR1, BLIL2, BLIR2, . . . are related, respectively, are input to latchcircuit 99.

FIG. 24 is a timing chart illustrating an operation of the completehidden refresh function-equipped DRAM shown in FIG. 23. A signal /HOLDis generated in a similar manner as in the first embodiment. SignalsφBL1 and φBL2 are inverted signals of internal clock signal int/RAS fora period during which respective memory blocks BK1 and BK2 are selected.Signals Pre.BS1 and Pre.BS2 correspond respectively to signals φBL1 andφBL2 latched by latch circuit 99. Specifically, signals Pre.BS1 andPre.BS2 correspond respectively to signals φBL1 and φBL2 of whichportions that coincide with an inverted signal of internal clock signalint/RAS are made constant at an “H” level. Signal BLIM swings atapproximately the same timing as internal clock signal int/RAS, and isoutput from control circuit 10. Signal Pre.BLI corresponds to signalBLIM latched by latch circuit 98.

Signal BLIR0 is at an “H” level (boosted potential Vpp) at all times.Each of signals BLIL1 and BLIR2 is an inverted signal of a logicalproduct signal of signals Pre.BS2 and Pre. BL1, and is normally at an“H” level (boosted potential Vpp) and attains an “L” level while memoryblock BK2 is selected. Signal BLIR1 is an inverted signal of a logicalproduct signal of signals Pre.BS1 and Pre.BLI, and is normally at an “H”level (boosted potential Vpp) and attains an “L” level while block BK1is selected.

While signal BLIR1 is at an “L” level, respective word lines WLs ofblock BK1 are sequentially selected, so that data refresh is carriedout. While signals BLIL1 and BLIR2 are at an “L” level, respective wordlines WLs of memory block BK2 are sequentially selected, so that datarefresh is carried out. Then, memory block BK3 is selected, and asimilar operation is carried out.

In the fourth embodiment, while one memory block BK (BK2, for example)is selected, a signal BLI (BLIL1 and BLIR2 in this case) will not bereset and held at an active level of an “L” level. Thus, powerconsumption is reduced compared to a conventional case where signal BLIis reset every time one word line WL is selected. Furthermore, sincevoltage Vpp of the amplitude of signal BLI is greater than power supplyvoltage VCC, the effect of reducing the power consumption by decreasingthe number of times of reset of signal BLI is significant.

Power consumption can further be reduced if the fourth embodiment iscombined with any of the first through third embodiments.

Fifth Embodiment

In the first through fourth embodiments described above, it has beenconfigured such that a prescribed internal control signal is not resetat each cycle in response to rising of internal chip enable signalint/CE from an “L” level to an “H” level. If the time during whichinternal chip enable signal int/CE is at an “H” level is short, however,the standby mode and the active mode will be changed in a short periodof time, which may lead to malfunction. Thus, in the fifth embodiment,in each of FIGS. 9, 16, 20 and 23, the signal for resetting flip-flop 94is changed from signal int/CE to a signal int/CED.

FIG. 25 is a circuit block diagram showing a configuration of a signalgenerating circuit 130 that generates signal int/CED. In FIG. 25, signalgenerating circuit 130 includes a delay circuit 131 and an AND gate 132.AND gate 132 receives signal int/CE and a delayed version of signalint/CE that is delayed in delay circuit 131 by a prescribed time (1 μs,for example), and outputs their logical product signal as signalint/CED.

When signal int/CE is raised from an “L” level to an “H” level, signalint/CED is raised from an “L” level to an “H” level after a lapse of 1μs. If signal int/CE is lowered from an “H” level to an “L” level before1 μs passes from rising of signal int/CE from an “L” level to an “H”level, then signal int/CED is unchanged and maintains an “L” level. Whensignal int/CE is lowered from an “H” level to an “L” level, signalint/CED is immediately lowered from an “H” level to an “L” level. Inthis manner, when internal chip enable signal int/CE attains an “H”level only for a short period of time, the standby state is maintained,which prevents occurrence of malfunction.

Sixth Embodiment

Further, in the first through fourth embodiments described above, it hasbeen configured not to reset row-related signals for refresh whileinternal chip enable signal int/CE is at an “H” level, and to reset therow-related signals for refresh while internal chip enable signal int/CEis at an “L” level. However, if internal chip enable signal int/CE islowered from an “H” level to an “L” level while the row-related signalsare active, determination as to whether the row-related signals are tobe reset or not is uncertain, which may cause malfunction. Thus, in thesixth embodiment, a transfer gate 135 and an inverter 136 are added tothe circuit shown in FIG. 6, for example.

Transfer gate 135 has one electrode receiving signal int/CE, and theother electrode connected to one input node of NAND gate 97. A rowactive signal /RA is directly input to a gate 135 a of transfer gate 135on the side of an N channel MOS transistor, and also input to a gate onthe side of a P channel MOS transistor via inverter 136. Row activesignal /RA is a signal indicating the period during which therow-related signals are activated, which is, e.g., an inverted signal ofsignal RX0-1.

FIG. 27 is a timing chart illustrating an operation of the circuit shownin FIG. 26. In the initial state, signals int/CE, /RA are both at an “H”level. Since signal /RA is at an “H” level, transfer gate 135 isrendered conductive, and a signal φA appearing at the other electrode oftransfer gate 135 is at an “H” level as is the signal int/CE. Whensignal /RA is pulled down to an “L” level at time t1, transfer gate 135is rendered nonconductive. Next, when signal int/CE is pulled down to an“L” level at time t2, signal φA is maintained at an “H” level, sincetransfer gate 135 is nonconductive. Next, when signal /RA is raised toan “H” level at time t3, transfer gate 135 is rendered conductive, andsignal φA is pulled down to an “L” level.

Accordingly, in the sixth embodiment, when internal chip enable signalint/CE is lowered from an “H” level to an “L” level (t2) during theperiod (t1 to t3) in which row active signal /RA is activated, therow-related signals are not reset immediately. The row-related signalsare reset only after row active signal /RA is inactivated (t3). This canprevent malfunction.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A semiconductor memory device having a complete hidden refreshfunction for performing data refresh without being externallyinstructed, comprising: a memory cell array having a plurality of memorycells arranged in rows and columns, a plurality of word lines providedcorresponding respectively to said rows, and a plurality of bit linepairs provided corresponding respectively to said columns, said wordlines being divided into a plurality of word line groups; an oscillatorfor outputting a clock signal having a predetermined first period; agroup selection circuit operative in synchronization with said clocksignal, and for sequentially selecting said plurality of word linegroups at a second period that is a multiple of said first period, thegroup selection circuit setting a group selection signal indicating theselected word line group to an active level and then resetting the sameto an inactive level at said first period in an active mode where dataread/write can be performed, and setting said group selection signal toan active level and then resetting the same to an inactive level at saidsecond period in a standby mode where data is retained; a word lineselection circuit for sequentially selecting a plurality of word linesbelonging to the word line group selected by said group selectioncircuit at said first period, the word line selection circuit setting aword line selection signal indicating the selected word line to anactive level and then resetting the same to an inactive level at saidfirst period; a word driver provided corresponding to each word line,and for setting the corresponding word line to a selected level whenboth of the corresponding group selection signal and the correspondingword line selection signal are set to the active level; and a refreshexecuting circuit for carrying out data refresh of each memory cellcorresponding to the word line set to the selected level by said worddriver.
 2. The semiconductor memory device according to claim 1,comprising: an instruction signal activating circuit for setting arefresh flag signal to an active level at a predetermined third period,and for setting said refresh flag signal to an inactive level inresponse to a refresh instruction signal attaining an active level; anda refresh instruction circuit for setting said refresh instructionsignal to the active level for a prescribed period of time in responseto said refresh flag signal attaining the active level in said standbymode, and for setting said refresh instruction signal to the activelevel for said prescribed period of time in response to said refreshflag signal attaining the active level and in response to completion ofdata read/write in said active mode, wherein said oscillator isactivated to output said clock signal when said refresh instructionsignal is set at the active level.
 3. The semiconductor memory deviceaccording to claim 1, further comprising a mode sensing circuit foroutputting a signal of a first level indicating that said semiconductormemory device is set to said standby mode when a chip enable signal isat an inactive level, and for outputting a signal of a second levelindicating that said semiconductor memory device is set to said activemode after a lapse of a predetermined time from a change of said chipenable signal from the inactive level to an active level, wherein saidgroup selection circuit sets said group selection signal to the activelevel and then resets the same to the inactive level at said firstperiod while said signal of the second level is output from said modesensing circuit, and sets said group selection signal to the activelevel and then resets the same to the inactive level at said secondperiod while said signal of the first level is output from said modesensing circuit.
 4. The semiconductor memory device according to claim1, further comprising a gate circuit receiving a chip enable signal, andallowing passage of said chip enable signal therethrough when said groupselection signal is at the inactive level, and prohibiting passage ofsaid chip enable signal when said group selection signal is at theactive level, wherein said group selection circuit sets said groupselection signal to the active level and then resets the same to theinactive level at said first period when the chip enable signal passedthrough said gate circuit is at an active level, and sets said groupselection signal to the active level and then resets the same to theinactive level at said second period when the chip enable signal passedthrough said gate circuit is at an inactive level.
 5. The semiconductormemory device according to claim 1, wherein the active level of saidgroup selection signal, the active level of said word line selectionsignal, and the selected level of said word line each correspond to aboosted potential higher than a power supply potential.
 6. Asemiconductor memory device having a complete hidden refresh functionfor performing data refresh without being externally instructed,comprising: a memory cell array divided into a plurality of memoryblocks, each memory block having a plurality of memory cells arranged inrows and columns, a plurality of word lines provided correspondingrespectively to said rows, and a plurality of bit line pairs providedcorresponding respectively to said columns; an oscillator for outputtinga clock signal having a predetermined first period; a word lineselection circuit operative in synchronization with said clock signal,and for sequentially selecting said plurality of word lines at a secondperiod that is a multiple of said first period, the word line selectioncircuit setting a word line selection signal indicating the selectedword line to an active level and then resetting the same to an inactivelevel at said first period in an active mode where data read/write canbe performed, and setting said word line selection signal to an activelevel and then resetting the same to an inactive level at said secondperiod in a standby mode where data is retained; a block selectioncircuit for sequentially selecting said plurality of memory blocks atsaid first period, the block selection circuit setting a block selectionsignal indicating the selected memory block to an active level and thenresetting the same to an inactive level at said first period; a worddriver provided corresponding to each word line, and for setting thecorresponding word line to a selected level when both of thecorresponding word line selection signal and the corresponding blockselection signal are set to the active level; and a refresh executingcircuit for carrying out data refresh of each memory cell correspondingto the word line set to the selected level by said word driver.
 7. Thesemiconductor memory device according to claim 6, comprising: aninstruction signal activating circuit for setting a refresh flag signalto an active level at a predetermined third period, and for setting saidrefresh flag signal to an inactive level in response to a refreshinstruction signal attaining an active level; and a refresh instructioncircuit for setting said refresh instruction signal to the active levelfor a prescribed period of time in response to said refresh flag signalattaining the active level in said standby mode, and for setting saidrefresh instruction signal to the active level for said prescribedperiod of time in response to said refresh flag signal attaining theactive level and in response to completion of data read/write in saidactive mode, wherein said oscillator is activated to output said clocksignal when said refresh instruction signal is set at an active level.8. The semiconductor memory device according to claim 6, furthercomprising a mode sensing circuit for outputting a signal of a firstlevel indicating that said semiconductor memory device is set to saidstandby mode when a chip enable signal is at an inactive level, and foroutputting a signal of a second level indicating that said semiconductormemory device is set to said active mode after a lapse of apredetermined time from a change of said chip enable signal from aninactive level to an active level, wherein said word line selectioncircuit sets said word line selection signal to the active level andthen resets the same to the inactive level at said first period whilesaid signal of the second level is output from said mode sensingcircuit, and sets said word line selection signal to the active leveland then resets the same to the inactive level at said second periodwhile said signal of the first level is output from said mode sensingcircuit.
 9. The semiconductor memory device according to claim 6,further comprising a gate circuit receiving a chip enable signal, andallowing passage of said chip enable signal therethrough when said wordline selection signal is at the inactive level, and prohibiting passageof said chip enable signal when said word line selection signal is atthe active level, wherein said word line selection circuit sets saidword line selection signal to the active level and then resets the sameto the inactive level at said first period when the chip enable signalpassed through said gate circuit is at an active level, and sets saidword line selection signal to the active level and then resets the sameto the inactive level at said second period when the chip enable signalpassed through said gate circuit is at an inactive level.
 10. Thesemiconductor memory device according to claim 6, wherein the activelevel of said word line selection signal, the active level of said blockselection signal, and the selected level of said word line eachcorrespond to a boosted potential higher than a power supply potential.11. A semiconductor memory device having a complete hidden refreshfunction for performing data refresh without being externallyinstructed, comprising: a memory cell array divided into a plurality ofmemory blocks, each memory block having a plurality of memory cellsarranged in rows and columns, a plurality of word lines providedcorresponding respectively to said rows, and a plurality of bit linepairs provided corresponding respectively to said columns; a refreshexecuting circuit provided between every two of said plurality of memoryblocks, and for carrying out data refresh of each memory cellcorresponding to a word line set to a selected level in the adjacentmemory blocks; an oscillator for outputting a clock signal having apredetermined first period; a block selection circuit operative insynchronization with said clock signal, and for sequentially selectingsaid plurality of memory blocks at a second period that is a multiple ofsaid first period, the block selection circuit setting a block selectionsignal indicating the selected memory block to an active level and thenresetting the same to an inactive level at said first period in anactive mode where data read/write can be performed, and setting saidblock selection signal to an active level and then resetting the same toan inactive level at said second period in a standby mode where data isretained; a word line selection circuit for sequentially selecting aplurality of word lines belonging to the memory block selected by saidblock selection circuit at said first period, the word line selectioncircuit setting a word line selection signal indicating the selectedword line to an active level and then resetting the same to an inactivelevel at said first period; a connection circuit provided correspondingto each memory block, and for connecting the corresponding memory blockto the corresponding refresh executing circuit and disconnecting theother memory block from the relevant refresh executing circuit while thecorresponding block selection signal is at the active level; and a worddriver provided corresponding to each word line, and for setting thecorresponding word line to a selected level when both of thecorresponding block selection signal and the corresponding word lineselection signal are set to the active level.
 12. The semiconductormemory device according to claim 11, comprising: an instruction signalactivating circuit for setting a refresh flag signal to an active levelat a predetermined third period, and for setting said refresh flagsignal to an inactive level in response to a refresh instruction signalattaining an active level; and a refresh instruction circuit for settingsaid refresh instruction signal to the active level for a prescribedperiod of time in response to said refresh flag signal attaining theactive level in said standby mode, and for setting said refreshinstruction signal to the active level for said prescribed period oftime in response to said refresh flag signal attaining the active leveland in response to completion of data read/write in said active mode,wherein said oscillator is activated to output said clock signal whensaid refresh instruction signal is set at an active level.
 13. Thesemiconductor memory device according to claim 11, further comprising amode sensing circuit for outputting a signal of a first level indicatingthat said semiconductor memory device is set to said standby mode when achip enable signal is at an inactive level, and for outputting a signalof a second level indicating that said semiconductor memory device isset to said active mode after a lapse of a predetermined time from achange of said chip enable signal from the inactive level to an activelevel, wherein said block selection circuit sets said block selectionsignal to the active level and then resets the same to the inactivelevel at said first period while said signal of the second level isoutput from said mode sensing circuit, and sets said block selectionsignal to the active level and then resets the same to the inactivelevel at said second period while said signal of the first level isoutput from said mode sensing circuit.
 14. The semiconductor memorydevice according to claim 11, further comprising a gate circuitreceiving a chip enable signal, and allowing said chip enable signal topass therethrough when said block selection signal is at the inactivelevel, and prohibiting passage of said chip enable signal when saidblock selection signal is at the active level, wherein said blockselection circuit sets said block selection signal to the active leveland then resets the same to the inactive level at said first period whenthe chip enable signal passed through said gate circuit is at an activelevel, and sets said block selection signal to the active level and thenresets the same to the inactive level at said second period when thechip enable signal passed through said gate circuit is at an inactivelevel.
 15. The semiconductor memory device according to claim 11,wherein the active level of said block selection signal, the activelevel of said word line selection signal, and the selected level of saidword line each correspond to a boosted potential higher than a powersupply potential.